Graded Exercise Testing in a Pediatric Weight Management Center: The DeVos Protocol

Abstract

Background:

In this article, we describe a protocol used to test the functional capacity of the obese pediatric patient and describe the peak oxygen consumption (VO_2peak) of patients seeking treatment at a pediatric weight management center.

Methods:

One hundred eleven (mean age, 12.5 ± 3.0 years) patients performed a multistage exercise test on a treadmill, of which 90 (81%) met end-test criteria and provided valid VO_2peak data. Peak VO₂ was expressed: (1) in absolute terms (L·min⁻¹); (2) as the ratio of the volume of oxygen consumed per minute relative to total body mass (mL·kg⁻¹·min⁻¹); and (3) as the ratio of the volume of oxygen consumed per minute relative to fat-free mass (mL·FFM·kg⁻¹·min⁻¹).

Results:

Mean BMI z-score was 2.4 ± 0.3 and the mean percent body fat was 36.5 ± 9.7%. Absolute VO_2peak (L·min⁻¹) was significantly different between sexes; however, relative values were similar between sexes. Mean VO_2peak was 25.7 ± 4.8 mL·kg⁻¹·min⁻¹ with a range of 13.5–36.7 mL·kg⁻¹·min⁻¹.

Conclusions:

Obese youth seeking treatment at a stage 3 pediatric weight management center exhibit low VO_2peak. The protocol outlined here should serve as a model for similar programs interested in the submaximal and peak responses to exercise in obese pediatric patients.

Introduction

Approximately 17% of US children and adolescents are obese.¹ Further, 3.8% are severely obese (BMI ≥99th percentile).² Based on the Expert Committee Recommendations for the staged approach to childhood obesity treatment,³ many of these youth are seeking treatment at multidisciplinary (stage 3) specialty clinics within children's hospitals.⁴ An important element of stage 3 pediatric weight management programs is the initial clinical evaluation. In general, there are a variety of assessments that occur during the initial (baseline) clinical visit(s).⁵ In a survey of stage 3 centers, the following assessments were considered: patient/family medical history; physical examination; blood pressure; body size and composition; blood chemistry; aerobic fitness; resting metabolic rate; muscle strength and flexibility; gross motor function; spirometry; sedentary behavior and physical activity (PA); dietary behavior and nutrition; and psychological assessments. Some domains of assessment (i.e., PA, diet, and blood chemistry) were more common than others. Specific to this article, only 44% of programs assessed aerobic fitness/physical working capacity. Protocols implemented varied from submaximal to maximal intensity and included both the cycle ergometer and treadmill (e.g., Astrand protocol, Bruce protocol, and so on).⁵

Most often used in pediatric cardiology and pulmonology,^6–8 exercise testing in the clinical setting is implemented for several reasons, including measurement of functional capacity or aerobic fitness level, assessment of exercise tolerance, disease diagnosis, determination of disease severity, and monitoring of the effects of treatment programs (medication, exercise training, and so on).⁶ Given that the goal of exercise testing is to evaluate how organs and systems linking pulmonary to cellular respiration perform under conditions of increased metabolic demand, the exercise testing protocol is typically progressive (i.e., exercise stages). Common testing modalities include treadmill walking and/or running or cycle ergometry.⁶ In principle, many distinct protocols can be used on either a treadmill or a cycle ergometer. Indeed, several protocols have been used to evaluate exercise performance and functional capacity in children.^9–11 Although some protocols are widely used (e.g., Balke,¹² Bruce,¹³ and so on), others are specific to laboratories for routine testing or specific to scientific investigations. Regardless, the exercise testing protocol should be selected based on the purpose of the test and the characteristics of the patient.

Despite the importance of assessing exercise capacity in a weight management setting, few articles have been dedicated to exercise testing of the obese child.¹⁴ Physical fitness and PA are each strongly associated with health, independently of weight loss.¹⁵ Obese children, in particular, may experience reduced health-related quality of life and physical function,¹⁶ which may, in turn, discourage participation in PA; therefore, it is important to evaluate exercise capacity in a controlled clinical setting in order to prescribe appropriate exercise interventions. In this article, we describe a protocol used to test the functional capacity of the obese pediatric patient and describe the peak oxygen consumption (VO_2peak) of severely obese patients seeking treatment at a stage 3 pediatric weight management center.

Methods

Patients

Participants were 143 obese patients who were seeking treatment at a pediatric obesity specialty clinic over a 1-year period. The target population at the Helen DeVos Children's Hospital Healthy Weight Center (Grand Rapids, MI) is obese children and adolescents who are patients of primary care providers and pediatric subspecialists practicing within the primary service region of west Michigan. Patients are referred to the center if they have a BMI ≥95th percentile according to the CDC BMI chart and the patient and family demonstrate the ability and commitment to engage in stage 3 weight management. All potential patients and their parent/guardian attend a multidisciplinary “consult visit,” which includes an initial patient/family assessment, program orientation, and determination of appropriate treatment course. After the first visit, a member of the clinical team discusses options for further treatment within the Healthy Weight Center. Treatment recommendations are based on each child and family's individual medical history and readiness or ability to make the necessary lifestyle changes. For eligible patients/families who initiate treatment, a second visit is scheduled during which additional patient characteristics are assessed, including the maximal exercise test. This retrospective chart review study protocol was approved by the human subjects institutional review board.

Laboratory Environment, Equipment, and Personnel

The physical space dedicated to exercise testing at the Healthy Weight Center approximates 280 square feet (14 × 20 feet) excluding adjoining office space, storage space, and bathroom. This space is a climate-controlled area with the temperature maintained in the range of 20–24°C (68–75°F). Major testing equipment includes a motorized treadmill (Trackmaster TMX425) and indirect calorimetry system (i.e., metabolic cart) (TrueMax 2400; ParvoMedics, Sandy, UT). A Monarch cycle ergometer (model 818E) is also available. For safety, a fall arrest harness is available (DBI Exofit; F.D. Lake Safety Equipment, Grand Rapids, MI) that attaches by a 24- or 42-inch long, 1-inch wide nylon lanyard to a support beam gantry (Spanco P/N 53-0001; F.D. Lake Safety Equipment) above the treadmill. In terms of emergency equipment, there is an oxygen tank in the laboratory and a crash cart with defibrillator and medications are available on the same floor. All exercise testing is conducted by a pediatric-trained exercise physiologist (i.e., specific coursework in pediatric exercise science) and assisted by a medical office assistant and/or exercise science undergraduate intern. All three individuals are cardiopulmonary resuscitation certified. Because we screen high-risk patients (see below), direct physician supervision is not required for low-risk testing.⁶ Nonetheless, one of the physicians of the Healthy Weight Center is immediately available (in the clinical suite) during exercise testing.

Pretest Screening

Before exercise testing, patients are screened for cardiovascular and pulmonary contraindications, as well as for orthopedic injuries and cognitive delays that may hinder testing. Patients with previously diagnosed cardiovascular conditions who have not been cleared by a physician are referred to the pediatric cardiology division for further evaluation before testing. Patients presenting triplicate blood pressures >95th percentile for age and height or pulmonary function values consistent with uncontrolled asthma are prescribed medications or referred to subspecialists for further evaluation before testing. When considering orthopedic and cognitive conditions, the exercise physiologist errs on the side of caution to ensure patient safety and eliminate any negative experience potentially associated with intense exercise.

Pretest Procedures

Our pretest procedures comply with recommendations provided in the American Heart Association statement on clinical exercise testing.⁶ In brief, the patient and family are provided information about the test before arrival. Patients are instructed to wear or bring comfortable exercise clothing, preferably shorts, T-shirt, and athletic shoes. When the patient arrives at the exercise laboratory for testing, a thorough explanation of the testing procedure is explained to the child and the parent. The child and parent are given the opportunity to have all questions answered about the test.

The patient is told that it is possible to terminate the test at any time even though he or she will be encouraged to continue to volitional fatigue and that the test generally lasts 8–12 minutes. The patient is given the opportunity to become familiar with the breathing apparatus (airtight mouthpiece) and headset. Any patient who lacks experience walking on a treadmill is given time to familiarize with treadmill walking before starting the test. Laboratory staff pay special attention to fostering confidence in the obese child during the pretest routine.

Protocol development

As previously mentioned, the protocol for an exercise test should reflect the purpose of the test and the characteristics of the patient being tested. Exercise testing obese pediatric patients in our center is not performed for diagnostic purposes (i.e., evaluating signs and symptoms aggravated by exercise or identifying abnormal responses to exercise). Rather, we aim to assess both the submaximal responses to exercise and establish a baseline for maximal functional capacity, which is reassessed post-treatment. In developing the protocol, several items were considered to accomplish our objectives to testing, including the mode of exercise, the exercise stimulus (e.g., speed and grade), and the duration of each exercise stage. We designed a continuous, multistage incremental protocol to be performed on a motorized treadmill. The treadmill was chosen as the mode of exercise given that walking is the major form of daily movement. Further, the initial submaximal speed of 2.5 mph was chosen because this speed falls within the range of self-selected walking speed for children.¹⁷ The initial stage is conducted at 0% grade for 4 minutes to reflect the physiological and perceptual responses of daily life (e.g., walking down the hallway in school). The 4-minute stage also allows for the assessment of steady-state submaximal responses. In order to complete the test in the recommended 8–12 minutes, both the speed and/or grade are increased based on estimates of VO₂ using the adult-derived American College of Sports Medicine equation as a guide.¹⁸ We also used VO_2peak values from previous studies to determine these stages. The DeVos graded exercise test protocol is shown in Table 1.

Table 1.

The DeVos Graded Exercise Test Protocol Using a Motorized Treadmill

Stage	Treadmill speed (mph)	Treadmill grade (%)	Duration (min)
1	2.5	0.0	4
2	3.5	5.0	2
3	3.5	7.5	2
4	3.5	10.0	2
5	4.5	10.0	2
6	5.5	10.0	(until exhaustion)

Execution of the exercise testing protocol

Subsequent to the pretest procedures explained above, the patient is fitted with the breathing apparatus and anchored to the fall arrest harness, if desired. The DeVos exercise testing protocol is executed with expired gases collected and analyzed by the indirect calorimetry system, calibrated before each testing session. Oxygen consumption is recorded in 15-second increments. Heart rate is measured continuously throughout the test using a Polar heart rate monitor. The treadmill is stopped when the patient can no longer continue exercising despite continued verbal encouragement. The time elapsed until exhaustion is used to indicate maximal treadmill time (minutes), and the highest value for oxygen consumption is considered VO_2peak. End-test criteria used to define whether subjects reach their true maximal oxygen consumption include at least three of the following five criteria: (1) visible evidence of exhaustion; (2) maximal heart rate ±200 beats/min; (3) respiratory exchange ratio ≥1.0; (4) Rating of perceived exertion = 10; and (5) a plateau in oxygen consumption (a change of <2 mL·kg⁻¹·min⁻¹ in VO₂ over the final 60 seconds).

Statistical Analysis and Expression of Peak VO₂ Values

Peak VO₂ is expressed three ways: (1) in absolute terms (L·min⁻¹); (2) as the ratio of VO₂ per minute relative to total body mass (mL·kg⁻¹·min⁻¹); and (3) as the ratio VO₂ per minute relative to fat-free mass (mL·FFM kg⁻¹·min⁻¹). Descriptive statistics for the total sample and for both sexes are displayed in a frequency distribution to emphasize the between-individual variability. An independent t-test examined the differences between sexes. All analyses were performed using SPSS software (version 18.0; SPSS, Inc., Chicago, IL).

Results

One hundred eleven (78%; age 6–17 years; mean age, 12.5 ± 3.0) of the 143 patients completed the exercise test. Of the 111 who performed the exercise test, 21 (19%) were excluded from the data analysis because of not meeting end-test criteria; only 1 of these children was under the age of 8 years, indicating that the protocol was well tolerated by young children. Hence, 81% (n = 90) of the patients performing the exercise test met end-test criteria and provided valid VO_2peak data.

Physical characteristics of the sample are shown in Table 2. In general, males were taller and heavier than females, although this was not statistically significant. Based on program criteria, all patients were obese; of which 65.5% were severely obese (BMI ≥99th percentile). Mean BMI z-score was 2.4 ± 0.3 and was significantly higher among males than females. Mean percent body fat determined by bioelectrical impedance was 36.5 ± 9.7%. Absolute VO_2peak (L·min⁻¹) was significantly different between sexes; however, when expressed per unit body mass or FFM values were similar between sexes. Figure 1 depicts the between-individual variation (13.5–36.7 mL·kg⁻¹·min⁻¹⁾ and mean group values (25.7 ± 4.8) for the conventional expression of VO_2peak in the total sample and by sex.

Figure 1.

Frequency distribution for VO_2peak in obese youth seeking treatment at a pediatric weight management center. VO_2peak, peak oxygen consumption.

Table 2.

Maximal Exercise Testing Performance in Obese Youth Seeking Treatment at a Pediatric Weight Management Center

	Male (n = 24)	Female (n = 66)	Total (n = 90)
Age (years)	12.5 (2.6)	12.5 (3.1)	12.5 (3.0)
Height (cm)	161.8 (15.9)	155.8 (12.1)	157.4 (13.4)
Weight (kg)	96.4 (31.5)	84.1 (24.8)	87.4 (27.1)
BMI (kg/m²)	35.8 (7.2)	34.0 (7.1)	34.5 (7.1)
BMI z-score	2.6 (0.3)^a	2.3 (0.3)	2.4 (0.3)
Body fat (%)	36.2 (9.7)	36.6 (9.7)	36.5 (9.7)
VO_2peak (L·min⁻¹)	2.6 (0.9)^a	2.1 (0.5)	2.2 (0.7)
VO_2peak (mL·FFM kg⁻¹·min⁻¹)	42.9 (7.3)	42.5 (14.4)	42.6 (12.8)
Time to exhaustion (min)	7.8 (1.7)	8.2 (1.8)	8.1 (1.8)

Significant difference between sexes (p < 0.05).

VO_2peak, peak oxygen consumption; FFM, fat-free mass.

Discussion

This article provides clinicians and researchers a detailed description of an exercise testing protocol used to test the functional capacity of the obese pediatric patient seeking treatment at a stage 3 weight management center. Various aspects are discussed, including the exercise testing process including protocol, expression of VO_2peak, comparison of VO_2peak to previous studies, and clinical utility.

Exercise Testing Protocol

We have developed and implemented an exercise testing protocol that allows for the determination of steady-state submaximal exercise responses at a walking speed that is typical of daily ambulation and also provides peak exercise values. The test is well tolerated by the patients, with over 80% achieving the maximal end-test criteria. This compares favorably with other research¹⁹ using similar protocols. Previously, Rowland²⁰ proposed a protocol for low-fit children that consisted of a constant treadmill speed (3 mph) and a slope that increased 2% every 2 minutes after the warm-up stage (3 mph, 3% grade). The current protocol may provide an alternative steady-state submaximal response that more closely represents typical daily ambulation (2.5 mph, 0% grade). The Healthy Active Living and Obesity Research Group developed a submaximal treadmill protocol for obese children during which children maintain a self-selected brisk walking pace as treadmill incline is increased.²¹ As with the protocol presented here, Breithaupt and colleagues note that the protocol was well tolerated by participants. However, we demonstrate that obese children are, in fact, able to attain maximal exercise testing criteria. Given the low VO_2peak values shown among severely obese adolescents, evaluation of responses to maximal or near-maximal levels is important, given that everyday activities, such as walking uphill, may require near-maximal effort.

Expression of Peak Oxygen Consumption

A major subject of debate in the pediatric exercise community is how to express VO_2peak values. Because absolute VO_2peak is a function of growth during childhood and adolescence, various approaches have been used to “normalize” and remove the influence of body mass. A detailed account of this issue and the approaches used to normalize VO_2peak is beyond the scope of this article, but can be found elsewhere.^22,23 Traditionally, body mass normalized VO₂ values have been expressed as a simple ratio standard (i.e., mL·kg⁻¹·min⁻¹). However, the theoretical and statistical limitations of the simple ratio standard have been widely addressed, yet largely ignored, in practice. Specifically, the simple ratio standard provides a statistical advantage for lighter individuals and penalizes heavier individuals.²⁴ This certainly has important implications for obese youth regardless of the functioning of the oxygen transport system, such that obese youth will appear to possess lower aerobic fitness simply as a function of the statistical limitations of ratio scaling. Nevertheless, the use of ratio standards remains common in exercise science given that it is easy to compute and comprehend. Alternatively, several statistical models, including analysis of covariance, allometric scaling, and multilevel modeling, have been used to create a “size-free” expression of VO_2peak.^22,23 Given that the major influence of body weight on VO_2peak is explained by FFM, another approach has been to calculate VO₂ per kg of FFM based on various body composition assessment methods. In the following section, we will compare our VO_2peak values expressed per kg of body mass and per kg of FFM to values from previous studies.

Comparison of Peak VO₂ Values to Previous Studies

Several previous studies have documented VO_2peak levels among overweight and obese youth.^25–33 In general, the mean relative VO_2peak obtained here is comparable to previous studies, with mean values ranging from 19 to 34 mL·kg⁻¹·min⁻¹. In a study of 48 severely obese (mean BMI = 45.5 ± 4.3 kg/m²) 8- to 18-year-old children and adolescents (mean age = 14.2 years), Gidding and colleagues³⁴ found the VO_2peak was 19 ± 5 mL·kg⁻¹·min⁻¹. Only 2 of 48 (4.1%) participants had a VO_2peak ≥30 mL/kg /min compared to 16 of 111 (14.4%) patients in the current study.

Mean values for estimated VO_2peak from the National Health and Nutrition Examination Survey for normal weight, overweight, and obese 12- to 18-year-old boys and girls were as follows: 48.2, 43.5, and 41.6 mL·kg⁻¹·min⁻¹ for boys, respectively, and 39.6, 37.6, and 35.9 mL·kg⁻¹·min⁻¹ for girls, respectively.³⁵ Mean VO_2peak values for boys and girls in this study are less than the 1st percentile of a nationally representative sample of US adolescents.³⁶ Experimentally, Cureton and colleagues³⁷ showed that treadmill time to exhaustion, VO_2max, and 12-minute run performance decreased linearly when increasing loads (5%, 10%, and 15% additional external weight) were worn on a belt. Thus, excess body fatness increases the “nonfunctional” load and has a negative influence on VO_2peak. Consequently, some researchers have expressed VO_2peak per unit FFM. In doing so, some, but not all, researchers have found that peak VO₂ is not significantly different between obese and nonobese after adjustment for FFM.^{18,20,22,24,25} However, a possible limitation to this approach is the assessment of body composition to determine FFM. Several methods are available and each has its own advantages and disadvantages.³⁸ Finally, reference values are not available for VO_2peak expressed as mL·FFM·kg⁻¹·min⁻¹.

Clinical Utility of the Exercise Test

A strength of this testing protocol is that it allows for use of data by the clinical team to evaluate the physiological and perceptual (i.e., rating of perceived exertion) responses to the submaximal stage at 2.5 mph, 0% grade. Results of treadmill testing are used in a variety of ways at the DeVos Healthy Weight Center. While conducting the exercise test, the exercise physiologist explains the concept of metabolic equivalents to help families understand intensities at which children should exercise. In the case of children who demonstrate very low exercise capacity, performance on the treadmill test is used as a basis to discuss the importance of regular exercise for improving functional capacity. Finally, many families express concern that their children “can't breathe” while exercising; in these cases, the treadmill test can be used to help build confidence that exercise is safe.

Limitations

Maximal exercise testing within clinical setting presents a few limitations. Although a treadmill is perhaps common in pediatric weight management clinics, the cost of an indirect calorimetry system can be prohibitive. Alternatively, stage 3 programs at children's hospitals may have access to a core pediatric exercise testing laboratory or one affiliated with pediatric cardiology, pulmonology, or physical therapy. To our knowledge, the gantry and harness system is unique to our clinic; however, it provides an important element of safety for the patient and clinician. Additionally, the time allotted for testing may be a deterrent to some programs. The total length of this visit is around 2.5 hours and includes several others measures and counseling conducted by the multidisciplinary team. To minimize the equipment and time required for exercise testing, future studies should examine the validity of predicting maximal values in obese patients. Finally, we currently do not conduct electrocardiography (ECG). We chose not to conduct ECG for two reasons: (1) The prescreening process considers cardiac abnormalities and (2) correct placement of electrodes on severely obese patients is difficult and may potentially increase the patient's anxiety associated with exercise testing. Clinical facilities with sufficient resources in staffing and equipment may consider implementation of ECG monitoring.

Summary and Conclusions

Given the extent of the pediatric obesity epidemic, several specialty clinics provide medical care for this patient population. Most of these clinics provide baseline assessments before initiating treatment. Exercise testing is an important assessment in clinical pediatric weight management programs and is becoming more common (personal communications). This article provides a detailed description of an exercise testing protocol used to test the functional capacity of the obese pediatric patient seeking treatment at a stage 3 pediatric weight management center and documents the low levels of VO_2peak in this population. The protocol and testing regimen outlined here should serve as a model for similar programs interested in examining the submaximal and peak responses to exercise in obese pediatric patients.

Footnotes

Acknowledgments

The authors thank the staff of the Helen DeVos Children's Hospital Healthy Weight Center. At the time of this work, the second author (E.H.G.) was completing a pediatric research fellowship supported by Grand Rapids Medical Education Partners.

Author Disclosure Statement

No competing financial interests exist.

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Graded Exercise Testing in a Pediatric Weight Management Center: The DeVos Protocol

Abstract

Abstract

Background:

Methods:

Results:

Conclusions:

Introduction

Methods

Patients

Laboratory Environment, Equipment, and Personnel

Pretest Screening

Pretest Procedures

Protocol development

Execution of the exercise testing protocol

Statistical Analysis and Expression of Peak VO2 Values

Results

Discussion

Exercise Testing Protocol

Expression of Peak Oxygen Consumption

Comparison of Peak VO2 Values to Previous Studies

Clinical Utility of the Exercise Test

Limitations

Summary and Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Statistical Analysis and Expression of Peak VO₂ Values

Comparison of Peak VO₂ Values to Previous Studies